day20 654.最大二叉树 617.合并二叉树 700.二叉搜索树中的搜索 98.验证二叉搜索树

654.最大二叉树 617.合并二叉树 700.二叉搜索树中的搜索 98.验证二叉搜索树

654.最大二叉树

class Solution {
    public TreeNode constructMaximumBinaryTree(int[] nums) {
        return findNode(nums, 0, nums.length);
        
    }   

    public TreeNode findNode(int[] nums, int leftIndex, int rightIndex) {
        if (rightIndex - leftIndex < 1) {// 没有元素了
            return null;
        }
        if (rightIndex - leftIndex == 1) {// 只有一个元素
            return new TreeNode(nums[leftIndex]);
        }
        int maxIndex = leftIndex;// 最大值所在位置
        int maxVal = nums[maxIndex];// 最大值
        for (int i = leftIndex + 1; i < rightIndex; i++) {
            if (nums[i] > maxVal){
                maxVal = nums[i];
                maxIndex = i;
            }
        }
        TreeNode root = new TreeNode(maxVal);
        root.left = findNode(nums, leftIndex, maxIndex);
        root.right = findNode(nums, maxIndex + 1, rightIndex);
        return root;
    }
}

617.合并二叉树

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null) return root2;
        if(root2 == null) return root1;

        root1.val += root2.val;
        root1.left = mergeTrees(root1.left, root2.left);
        root1.right = mergeTrees(root1.right, root2.right);

        return root1;
    }
}

700.二叉搜索树中的搜索

class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        if(root == null) return null;
        if(root.val > val) {
            return searchBST(root.left, val);
        }else if(root.val < val){
            return searchBST(root.right, val);
        }else{
            return root;
        }
    }
}

98.验证二叉搜索树

class Solution {
    TreeNode pre;
    public boolean isValidBST(TreeNode root) {
        if(root == null) return true;
        boolean rootLeft = isValidBST(root.left);
        if(pre != null && root.val <= pre.val){
            return false;
        }
        pre = root;
        boolean rootRight = isValidBST(root.right);

        return  rootLeft&&rootRight;
    }
}